The present invention relates to an instantaneous overcurrent relay, and more particularly to a static instantaneous relay with low transient overreach.
As a result of advancements in solid state technology, static protective relays have been developed. Exemplary static relays are disclosed in U.S. Pat. No. 4,092,690, entitled, "Protective Relay Circuit Providing a Combined Distance and Overcurrent Function", issued May 30, 1978; and in U.S. Pat. No. 4,034,269, entitled, "Protective Relay Circuits", issued July 5, 1977.
An overcurrent protective relay is employed when it is necessary to trip a circuit breaker when greater than a predetermined magnitude of current flows into a particular portion of a power system. Such overcurrent relays are broadly characterized as "instantaneous" or "time delay", with "time delay" being further characterized as "inverse"; "very inverse"; or "extremely inverse". An instantaneous overcurrent relay is generally provided with no intentional time delay whereas a time delay overcurrent relay is provided with various degrees of time delay.
Instantaneous overcurrent relays have a tendency to overreach. Overreach is generally defined as the tendency of a relay to pick up, i.e., operate, for fault currents less than one would expect; that is, if the effect of offset in the fault current wave were neglected. Percent overreach is a term which describes the degree to which this transient tendency exists, and may be defined as follows:
Percent overreach=100 (A-B)/A, PA1 A=relay pickup current, in steady state rms amperes. PA1 B=the steady-state rms amperes which, when fully offset initially, will just pick up the relay. PA1 (100.times.10)/(100-15) amperes=11.8 amperes
where
As an example of the foregoing, consider an application involving electro-mechanical instantaneous overcurrent relays having a typical 15% transient overreach. For a fault with a steady state component of 10 amps, if the relay is not to operate for such a fault, the value A, i.e., the relay pick-up setting, must exceed:
However, referring now to the static instantaneous overcurrent relay, it is to be appreciated that such static relays operate much more quickly than the electromechanical instantaneous overcurrent relays. This is due in large measure to the fact that the electromechanical relays have a certain amount of inertia to overcome before operation whereas no such inertia must be overcome in a static relay. Accordingly, the transient overreach percentage for a static relay approaches the maximum value which is the value for a relay which is fast enough to respond to the instantaneous magnitude of the current. For example, in an exemplary power line having a line angle (.theta.) of 75.degree., the maximum transient overreach percentage approaches 33%. This transient overreach percentage can be responded to with the technique discussed above in the case of the 15% electromechanical transient overreach percentage. However, in the case of the 33% transient overreach situation, this technique is undesirable as it requires the pickup of the relay to be set 50% greater than the steady state fault current. Such a setting would be contrary to good protective relaying engineering practices which generally require that the relay be set as close to the steady state load current as possible because this permits more complete coverage of the line under fault conditions.
One proposed technique to employ in connection with this greater transient overreach problem in static overcurrent relays is to delay the operation of the relay for a time period sufficient to reduce the transient overreach percentage to an acceptable level. However, this technique is undesirable because excessive delays would be injected to cover all conditions.
One application for a static instantaneous overcurrent relay is for protecting an electrical power distribution line. Such a distribution line may have a typical line angle (.theta.), the angle between the system voltage and system current, of less than about 77.degree.. It would be desirable to provide such a static instantaneous overcurrent relay having a transient overreach of less than 10% for line angles up to about 77.degree..
Accordingly, it is a general object of this invention to provide a static instantaneous overcurrent relay with low transient overreach.
It is another object of this invention to provide such a static instantaneous overcurrent relay having a transient overreach percentage of less than 8% for a protected line angle (.theta.) up to 75.degree..
It is another object of this invention to provide such a static instantaneous overcurrent relay having a transient overreach percentage of less than 8% for a protected line angle up to 75.degree. with a three phase input signal and only one measuring unit.